Mobile device antennas have limited bandwidth. But increasingly, mobile devices, or mobile connectivity systems for portable devices, serve as primary communication devices. These devices, which include PDAs such as BlackBerry and notebook computers equipped with mobile connectivity cards, must handle relatively high bandwidth communications such as IMAP email, graphical web browsing, and the like, not to mention bandwidth intensive applications such as video streaming or IP telephony. Further, traditional mobile devices increasingly serve as sites of high-bandwidth activity such as video streaming, media messaging, and the like.
To support high bandwidth applications over mobile networks, mobile devices increasingly require innovative antennas that permit high bandwidth traffic over existing mobile communications infrastructure. Examples include the Enhanced Data Standard for GSM Evolution (EDGE), the General Packet Radio Service (GPRS) standard, and the Universal Mobile Telecommunications Standard (UMTS). These and others attempt to adapt new data needs to legacy wireless communications infrastructure including the global system for mobile communications (GSM850) or extended GSM (EGSM), the digital communication system (DCS), the personal communication system (PCS), and wide-band code-division multiple access (WCDMA). These attempts further strain antenna design—already subject to safety, cost, and size requirements or regulations—by requiring multiband or broadband resonance. Traditional antenna designs are unable to meet these requirements and hence alternative approaches are needed.
Planar antennas have features of low cost, low profile and light weight. Planar antenna performance depends, among other things, on the shape and dimensions of the antenna and slits or slots on ground planes. FIGS. 1 to 3 illustrate known configurations of planar inverted-F antennae (PIFA), all of which have an operating frequency band centered around a characteristic frequency.
FIG. 1 shows a planar inverted-F antenna (PIFA) antenna 100 comprising a planar electrically conductive radiating element 101, electrically conductive ground plane 102 parallel to the radiating element 101, and, connecting these two, a ground contact 103. The feed electrode 104 permits connection of the radiating element 101 to an antenna port of a radio apparatus (neither shown). The upper elements 101, 103, and 104 of the PIFA 100 are typically manufactured by progressive stamping processes applied to thin sheet metal. The lower ground plate is typically embodied as a plated area on the surface of a printed circuit board (PCB), which facilitates electrical coupling between the PCB and the upper elements of the PIFA.
FIG. 2 shows a PIFA structure 200 in accordance with European Patent Document No. 484,454 that is built around a dielectric body 204. The antenna consists of a radiating element 201, ground plane 202 and ground contact 203, each of which are plated onto the body 204. In this design, a feed element 205 electromagnetically coupled to the radiating element 201 feeds the antenna. The structure is mechanically sturdy, but the dielectric body block makes it relatively heavy. Further, the dielectric body narrows the impedance bandwidth of the antenna and degrades the radiation efficiency as compared to an air-insulated PIFA structure.
FIG. 3 shows a PIFA structure 300 structured around a radiating element 301. The radiating element 301 is generally rectangular, but forms a gap 302. The portions of the radiating element 301, including the strip 305, form an extended structure with an increased electrical length relative to a rectangle of the same size. This modification lowers the antenna's characteristic frequency.
However, these PIFA structures are not designed to fit in a small confined space while communicating efficiently in a wide frequency band.
One known class of PIFA designs provide increased bandwidth through a switchable antenna arrangement. These PIFA include a parasitic element that is connectable to a main radiator to alter the electrical length of the radiator and thus provide multiple frequency tuning for the antenna. For example, Milosavljevic in US Patent Application 2004/0207559 A1 describes a PIFA with a conductive parasitic element switchably coupled to ground, which alters the antenna's tuning when coupled to ground. When grounded, the parasitic element provides additional capacitance to the high-band resonator, which changes the electrical length of the high-band slot radiator and tunes the resonance frequency higher. Grounding the parasitic element also affects the tuning in the low-band slot. When grounded, the loading effect of the parasitic element is changed and thus changes the tuning of the low band slot.
A main drawback of this solution is that loading the radiator causes dissipation and reduces efficiency. Furthermore, many implementations of this concept require multiple switching elements, including in the matching circuitry for the antenna, which further reduce efficiency and add expense.